Accelerating cosmologies and the phase structure of F ( R ) gravity with Lagrange multiplier constraints: A mimetic approach

We study mimetic F(R) gravity with a potential and Lagrange multiplier constraint. In the context of these theories, we introduce a reconstruction technique which enables us to realize arbitrary cosmologies, given the Hubble rate and an arbitrarily chosen F(R) gravity. We exemplify our method by realizing cosmologies that are in concordance with current observations (Planck data) and also well-known bouncing cosmologies. The attribute of our method is that the F(R) gravity can be arbitrarily chosen, so we can have the appealing features of the mimetic approach combined with the known features of some F(R) gravities, which unify early-time with late-time acceleration. Moreover, we study the existence and the stability of de Sitter points in the context of mimetic F(R) gravity. In the case of unstable de Sitter points, it is demonstrated that graceful exit from inflation occurs. We also study the Einstein-frame counterpart theory of the Jordan-frame mimetic F(R) gravity, and we discuss the general properties of the theory and exemplify our analysis by studying a quite interesting (from a phenomenological point of view) model with two scalar fields. We also calculate the observational indices of the two-scalar-field model, by using the two-scalar-field formalism. Furthermore, we extensively study the dynamical system that corresponds to the mimetic F(R) gravity, by finding the fixed points and studying their stability. Finally, we modify our reconstruction method to function in the inverse way and thus yield which F(R) gravity can realize a specific cosmological evolution, given the mimetic potential and the Lagrange multiplier.